It is desirable to develop a process for biological treatment of organic wastewater that can be employed for the treatment of organic wastewater having a wide range of contaminant concentrations, such as domestic wastewater, sewage, wastewater from food plants and pulp plants. In particular, it is desirable to develop a process for biological treatment of organic wastewater in which treatment efficiency can be improved and the amount of excess sludge generated can be reduced without degrading the quality of the treated water.
An activated sludge process used in the biological treatment of organic wastewater is widely used for sewage treatment, industrial wastewater treatment, and the like because this process is advantageous in that, for example, the quality of the treated water is satisfactory and maintenance can be easily performed. However, the biochemical oxygen demand (BOD) volume loads in the activated sludge process range from about 0.5 to 0.8 kg/m3/d, and thus a large site area is necessary. Since 20% of the decomposed BOD is converted into bacterial cells, that is, sludge, a large amount of excess sludge must be treated.
A fluidized bed process in which a carrier is added is known as a high-load treatment of organic wastewater. In this process, the operation can be performed at a BOD volume load of 3 kg/m3/d or more. However, in this process, the amount of generated sludge corresponds to about 30% of the decomposed BOD, which is larger than that in the existing activated sludge process.
According to a process disclosed in Japanese Examined Patent Application Publication No. 56-48235, first, organic wastewater is treated with bacteria in a first treatment tank. In this treatment, organic substances contained in the wastewater are subjected to oxidative decomposition and are converted to bacterial cells of non-flocculating bacteria. Subsequently, the bacterial cells are removed by being preyed upon by sessile protozoa in a second treatment tank. This process reduces the amount of excess sludge. Japanese Examined Patent Application Publication No. 62-54073 also describes a similar two-stage biological treatment. These processes can realize a high-load operation and increase the efficiency of activated sludge treatment.
According to a description in Japanese Patent No. 3360076, in such a process of a two-stage biological treatment, activated sludge containing protozoa is drained from a biological treatment tank, subjected to sterilization and a solubilization treatment in a reaction treatment tank, and returned to the biological treatment tank, thereby further reducing the amount of excess sludge generated.
According to a process described in Japanese Patent No. 3410699, the former stage biological treatment is performed by means of a carrier-fluidized bed process and the latter stage biological treatment is performed by means of a multistage activated sludge process, thereby further reducing the amount of excess sludge generated. In this process, the activated sludge treatment in the latter stage is operated at a low load, a BOD sludge load of 0.1 kg-BOD/kg-mixed liquor suspended solids (MLSS)/d. Thus, the sludge is subjected to auto-oxidation to markedly reduce the amount of sludge drained.
According to Japanese Examined Patent Application Publication No. 55-20649, first, organic wastewater is treated with bacteria in a first treatment tank. In this treatment, organic substances contained in the wastewater are subjected to oxidative decomposition and are converted to bacterial cells of non-flocculating bacteria. Subsequently, the bacterial cells are removed by being preyed upon by sessile protozoa in a second treatment tank, thereby reducing the amount of excess sludge. Furthermore, this process can realize a high-load operation and increase treatment efficiency of the activated sludge process.
For example, Japanese Unexamined Patent Application Publication No. 2000-210692 proposes a countermeasure for a degradation of treatment performance caused by a fluctuation of the quality of raw water, which is a problem in the process disclosed in Japanese Examined Patent Application Publication No. 55-20649. Specific processes thereof include “a process in which the fluctuation of the BOD of water to be treated is controlled within 50% from the median of the average concentration”, “a process in which the qualities of water in a first treatment tank and first treated water are measured with time”, and “a process in which, when the quality of the first treated water deteriorates, a microbial preparation or seed sludge is added to the first treatment tank”.
According to Japanese Examined Patent Application Publication No. 60-23832, when bacteria, yeast, actinomycetes, algae, mold, primary sedimentation sludge, or excess sludge of the wastewater treatment is preyed upon by protozoa or metazoa, the floc size of the above bait is reduced to be smaller than the mouth of the protozoa or metazoa by ultrasonic treatment or mechanical agitation.
The above-described multistage activated sludge process involving a predation effect caused by microorganisms has already been in practical use for the treatment of organic wastewater. For some types of target wastewater, this process can improve treatment efficiency and reduce the amount of sludge generated.
For example, when the process involving predation by animalcules is used, although the effect of reduction in the amount of sludge generated differs depending on process conditions or the quality of the wastewater, it is believed that this process can reduce the amount of sludge by about 30% to 70% of that generated in an existing activated sludge process.
However, although the effect of reduction in the amount of sludge generated differs depending on process conditions or the quality of the wastewater, the amount of sludge reduced in the above process is about half of the amount of sludge that is generated in a single-tank activated sludge process. The reason for this is that, in the microorganism tank used in the latter stage in which sludge that mainly contains bacteria is preyed upon, most of the sludge is not preyed upon and remains or the microorganisms that perform the predation cannot be maintained at high concentrations.
Furthermore, since the animalcules that perform the predation are higher living organisms than bacteria, the life-span of the animalcules is long (the rate of autolysis of the animalcules is low). This makes it more difficult to reduce the amount of sludge.
In addition, some animalcules proliferate by division and some animalcules proliferate via eggs. A typical example of the latter metazoa is rotifers. It is believed that such animalcules contribute to the reduction in the amount of sludge. However, these animalcules are not always in a state of proliferation (in a state capable of laying eggs). Over ten days after hatching, the animalcule does not lay eggs. After another over ten days pass, the animalcule dies naturally. In addition, when these animalcules are dominant, even when the animalcules lay eggs, in the case where a sufficient number of imagoes exist or in the case where a large portion of sludge is composed of dung pat and the amount of bacteria serving as bait is small, the eggs cannot hatch in some cases. Consequently, the animalcules in the tank may die all at once. In the activated sludge process using animalcules, such a characteristic of metazoa makes it difficult to stably maintain the generation of a small amount of sludge for a long period of time.
In the multistage activated sludge process involving the autolysis of bacteria, regardless of the state of the bacteria (a dispersed state, a floc state, or a filamentous state), the amount of sludge generated can be reduced by about 50% of that generated in an existing activated sludge process. However, in order to achieve a 50% decrease in the amount of sludge generated, in the biological treatment tank in which the autolysis is performed, the sludge retention time must be set to a large value. Therefore, when a membrane separation device is introduced, the maintenance cost for the membrane cancels out the reduction in the operating cost due to the decrease in the amount of sludge generated.
Accordingly, in order to achieve a decrease of 50% or higher in the amount of sludge generated using an existing aerating tank with a low operating cost, it is effective to use animalcules. In order to stably achieve this, it is necessary to stably produce bacteria that are easily preyed upon by the animalcules in the biological treatment tank used in the former stage.